I just installed both Max 5.1.7 and the new audiounit~
Only 2 plugins work - AUPitch (Apple) and Fire.
All of the others just freeze some of the audio file playing through (or with the original help file, play the tone, un-pulsed from the tri~ 220 object in the [p pulse-generator]). Bypass has no effect.
I removed everything from the Components folder & restarted Max so only the Apple AUs appeared (where are they stored, btw?) - same thing. Only AUPitch works. I put the components back again. Same again.
I don't think I understand. The "getpluginnames" message should return all of the plug-ins available for a given channel configuration. While the plug-in scan is running this list may be incomplete. Perhaps this what you are seeing?
If you have a particular synth plug-in that is not providing you with preset names, then please let me know what plug-in it is and we can try to follow-up from there.